A group III nitride single crystal such as typically gallium nitride (GaN) is useful as a substance applicable to light-emitting devices such as light-emitting diodes, laser diodes, etc., and high-frequency high-power electronic devices such as HEMT, HBT, etc. Accordingly, it is needed to efficiently produce a group III nitride semiconductor crystal such as GaN or the like of high crystallinity.
For example, a most ideal substrate usable for growth of a GaN crystal thereon is a GaN substrate. However, the nitrogen equilibrium vapor pressure over GaN is extremely high as compared with that over Ga, and therefore, it is difficult to grow a bulk crystal of the compound according to a conventional melt growth method or the like. Accordingly, there is employed a method of producing a GaN crystal, which comprises growing a GaN crystal on a substrate formed of a material differing from GaN, or that is, a substrate formed of a different material (e.g., sapphire substrate, SiC substrate, Si substrate, GaAs substrate, etc.), followed by removing the different material substrate.
At present, the most general GaN substrate is a GaN substrate having a C-plane as the main plane thereof, for which is known a method comprising growing a thick GaN layer having a thickness of at least 300 μm through HVPE (hydride vapor phase epitaxy), then removing the base substrate, followed by slicing and/or polishing the GaN free-standing crystal to give a GaN substrate having a C-plane as the main plane thereof. The main plane means a plane on which a device is formed, or the broadest plane of a structure.
In InGaN-based blue and green LED and LD comprising a GaN substrate in which the C-plane is the surface thereof, there is a problem in that a piezoelectric field forms in the growth axis direction, or that is, in the C-axis direction thereof. The piezoelectric field forms as a result of piezoelectric polarization owing to the crystal structure deformation of the InGaN layer, and the polarization separates the holes and the electrons injected into the light-emitting layer, thereby lowering the probability of recombination that contributes toward light emission. As a result, the internal quantum efficiency lowers, therefore reducing the external quantum efficiency in light-emitting devices. For retarding the influence of the piezoelectric field mentioned above, studies of InGaN-based blue and green LED and LD are being prosperous, in which the non-polar plane referred to as an A-plane or an M-plane that is perpendicular to the C-plane of a GaN crystal is the growth plane thereof (Non-Patent Reference 1).
In that situation, there is increasing a demand for a group III nitride semiconductor substrate such as GaN substrate or the like which has a large area and has good crystallinity, which has a low defect density and has high quality, and which has a surface of a non-polar plane.
Patent Reference 1 discloses a substrate having a plane perpendicular to the C-plane thereof, for example, a (01-10) plane or (11-20) plane as the main plane thereof. However, Patent Reference 1 does not disclose a method for producing substrates.
Patent Reference 2 describes a method comprising growing a crystal on a C-plane in the C-axis direction followed by slicing the crystal along the plane parallel to the growth direction, saying that the dislocation line of the sliced crystal extends in the growth direction.
Patent Reference 3 discloses a technique of aligning plural nitride semiconductor bars each having a C-plane and an M-plane in such a manner that the C-planes could face each other with the M-plane facing up, and growing a nitride semiconductor on the M-plane.
Patent Reference 4 discloses a technique of growing GaN on the A-plane or the M-plane of a seed crystal GaN in a melt followed by further growing GaN in the −C-axis direction.    Patent Reference 1: JP-A 10-335750    Patent Reference 2: JP-A 2002-29897    Patent Reference 3: JP-A 2006-315947    Patent Reference 4: JP-A 2006-160568    Non-Patent Reference 1: 27th Basic Lecture of Thin Film/Surface Physics (issued on Nov. 16, 1998), Society of Applied Physics of Japan, Subcommittee of Thin Film/Surface Physics, p. 75